Since natural leather is superior in durability and air permeability, it has been used in various articles, such as clothes, shoes, and the like, to take advantage of these properties. However, since natural leather is expensive, a leather-like sheet structural material is developed as a substitute for natural leather. At present, the leather-like sheet structural material is widely used as artificial leather in clothes, shoes, and the like.
However, since artificial leather is produced by forming a film layer on a sheet structural material comprising a supporting fabric and a porous layer formed on the supporting fabric, it is inferior in air permeability and feeling compared to natural leather at present. To solve this problem, the present inventors previously invented artificial leather having feeling and air permeability similar to those of natural leather (Japanese Patent Kokai Publication No. 8-232174). Artificial leather described in this publication is characterized in that a concavo-convex portion is formed on the surface of a sheet structural material having a porous layer, and a film layer is formed only on the convex portion of the concavo-convex portion of the surface, thereby forming a leather-like appearance. With this constitution, since a film layer is not formed on the concave portion of the surface of the porous layer, good air permeability and appearance like leather are provided.
According to such a constitution, there can be obtained artificial leather having the same air permeability as that of natural leather. Natural leather and artificial leather are durable, but are inferior in air permeability to a normal cloth. Therefore, when they are used in shoes, the air permeability is inferior to that of shoes made of a cloth. Accordingly, it is envisioned that usage of artificial leather can be increased by developing a sheet structural material wherein the air permeability is further improved while maintaining an appearance similar to natural leather.
The porous layer to be formed on the supporting fabric has hitherto been formed by applying a coating solution containing a base resin and dimethylformamide (hereinafter abbreviated to "DMF") as a solvent on the supporting fabric and dipping in water to remove DMF. According to this method, environmental pollution problems arose, such as wastewater disposal, recovery of DMF, and the like. Furthermore, since the method using DMF includes a step of drying after dipping in water, it was necessary to practice a step which is complicated and requires large energy consumption.
An object of the present invention is to solve these problems of the prior art, and to provide a sheet structural material and a leather-like sheet structural material, which are suited to produce artificial leather having air permeability superior to natural leather, and to a method of producing the same. Another object of the present invention is to provide a sheet structural material and a leather-like sheet structural material, which can be produced by a comparatively simple production step without causing environmental pollution problems, and to a method of producing the same.